In many manufacturing applications, a continuous web is utilized as a substrate on which is deposited at least one layer, the web being commonly constructed out of a thin, flexible material, such as paper, plastic, foil, glass or a composite thereof. For example, one or more layers of film are commonly deposited, etched, embossed and/or printed onto a common web of thin, flexible material to produce, inter alia, microelectronic devices, microoptical devices and pharmaceuticals.
Web handling systems are well known in the art and are commonly used to transport, redirect or otherwise handle various types and thicknesses of webs throughout a manufacturing process. Web handling systems often rely upon an arrangement of rollers, such as nip or pinch rollers, to assist in the transport of webs between processing stations.
Throughout various manufacturing industries, higher web processing speeds are continuously sought in order to maximize productivity. However, one of the significant obstacles in improving web processing speeds is that web handling systems often rely upon commercially available rollers that experience direct solid-to-solid contact between moving parts. It is to be understood that web handling systems are required to apply a pulling force on a web to generate and maintain motion. The tension which must be applied to the web to affect its transport must be sufficient enough to overcome the internal friction experienced by the non-driven, or idler, rollers as well as the drag applied to the web during various processing steps (e.g., viscous drag resulting from the application of a coating). However, this threshold of tension that needs to be applied to the web can only be controlled to the extent that the level of friction for each mechanical roller is known.
Because the level of friction experienced by traditional mechanical rollers is not precisely determinable, excess tension is often applied to the web during transport which, in turn, can result in considerable distortion and stress to the web. With very sensitive webs, this distortion and stress can result in various types of irreversible damage, such as stress birefringence, crack propagation or, in certain circumstances, complete breakage of the web, which is highly undesirable.
Accordingly, the tension applied to a fragile running web needs to be precisely controlled to limit the likelihood of performance deterring effects. In the art, web tension control is most accurately achieved using one or more frictionless, or near frictionless, rollers in place of traditional mechanical rollers. Without internal roller friction, the web handling system can be designed to apply only the critical force that is required to transport the web through its various manufacturing processes, which is highly desirable. More specifically, by utilizing frictionless rollers, the tension applied to the web can be controlled by simply regulating the speed and torque of each motor that drives the system, thereby enabling the tension applied to the web at a certain processing station to be independently controlled by modifying the torque of the motors that drive the web through the particular processing station.
Frictionless rollers often incorporate the principal design characteristics of fluid film bearings to achieve contact-free movement between parts. A fluid film bearing, also commonly referred to in the art simply as a fluid bearing, is a machine part that is adapted to support a load. Traditionally, a fluid bearing includes two or more adjacent parts that rotate or otherwise move in relation to one another. A thin layer of liquid or gas is delivered into a nominal gap defined between opposing faces of the two or more parts. As a result, relative motion can be achieved between the parts in a contact-free manner. The lack of direct contact between moving parts of a frictionless roller is beneficial in not only limiting the tension applied by the roller to fragile webs during handling but also minimizing component wear, limiting heat generation and providing high stiffness capabilities.
One type of frictionless roller which is commonly utilized in web handling systems is constructed using the principal design features of a cylindrical air bearing. A cylindrical air bearing includes a cylindrical inner component that that extends coaxially within a cylindrical outer component, the inner and outer components being dimensioned such that a small, uniformly spaced gap is defined therebetween into which fluid can be delivered.
For example, in U.S. Pat. No. 6,641,513 to J. K. Ward, the disclosure of which is incorporated by reference, there is shown a low inertia, low friction roller, which is particularly adapted for handling relatively high speed, relatively fragile running webs. The roller comprises an inner tube that is disposed substantially coaxially about an inner tube and is rotatable with respect to the inner tube. An annular gap is defined between the inner and outer tubes and has a first portion that is supplied with a restricted flow of a pressurized compressible fluid and that is adjacent the portion of the outer tube about which the web passes. A second portion of the annular gap is circumferentially spaced from the first portion of the annular gap and communicates with a fluid exhaust passage in the inner tube. The dimensions of the annular gap are selected so that the fluid pressure in the first portion is greater than the fluid pressure in the second portion and so that the pressure of the fluid in the first portion of the annular gap will substantially balance the force exerted by the web on the outer tube as the web passes about the outer tube.
Although well-known in the art, rollers of the type described in the '513 patent that rely upon the principal design features of a cylindrical air bearing have been found to suffer from a notable drawback. Specifically, certain performance characteristics associated with such rollers, such as load capacity and stiffness, are largely defined by particular geometric properties of the roller that include, among other things, the angle and surface area of the opposing gap defining surfaces, the spacing of the gap, and the orifice size of its fluid delivery channels. Since these geometric properties are permanently defined, or fixed, upon completion of the manufacture of the individual components, adjustability of fluid bearing performance characteristics is typically unattainable once machined.
As a consequence, the degree of precision that is required in machining the various components of a cylindrical bearing roller has been found to be of critical importance. However, it is to be understood that because many of the determining dimensions are considerably small by design, the machining precision that is required cannot be reliably obtained (e.g., due to normal machining tolerances). In particular, it should be noted that fluid bearings are commonly designed with a gap spacing that is exceptionally small (often approximately 0.00015 times the radius of the inner component) in order to increase stiffness and thereby maximize performance. As a result of the challenges associated with accurately machining parts that define an exceptionally small gap, rollers of the type described in the '513 patent are commonly constructed using a larger scaling factor in order to achieve the stiffness and load capacity required to handle high speed and fragile running webs. However, this significant increase in overall roller size renders it considerably expensive to manufacture and rather difficult to integrate into relatively compact web-handling systems.
In view of the aforementioned shortcoming, frictionless rollers are also commonly constructed using the design characteristics of tapered, or non-cylindrical, fluid bearings, such as conical bearings. A conical bearing is similar to a cylindrical bearing in that a conical bearing includes an inner component that that extends coaxially within an outer component, the inner and outer components being dimensioned such that a small, uniformly spaced gap is defined therebetween into which fluid can be delivered. A conical bearing differs from a cylindrical bearing in that the inner component of a conical bearing includes a generally conical portion that aligns within a corresponding conical cavity formed in the outer component. Due to the angled, or tapered, design of opposing surfaces in a conical bearing, gap spacing can be adjusted by axially displacing the conical inner component relative to the outer component.
However, it is to be understood that the wedge-shaped interrelationship between opposing surfaces of a conical bearing creates a net axial force due to the hydrodynamic action of the fluid, the net axial force resulting in the axial displacement of one component relative to the other component. In other words, the delivery of fluid between the opposing tapered surfaces that define the gap creates a net axial force that naturally separates the opposing parts.
In response thereto, conical fluid bearings typically rely upon on design symmetry to counteract the axial forces that would ordinarily result in component separation. Specifically, a conical bearing is traditionally designed to include an inner component that includes a pair of mirror-image conical members that are coaxially joined, the inner component extending axially within an outer component that is shaped to include a similarly designed cavity for receiving the inner component. As can be appreciated, the inclusion of opposing, mirror-image, conical members yields a pair of axial forces that directly counterbalance, or cancel, each other. As a result, the net axial force for a fluid bearing that includes a pair of opposing, mirror-image conical members is effectively eliminated.
Although well known in the art, conical bearings that include a pair of opposing, mirror-image conical members have been found to suffer from a number of notable shortcomings.
As a first shortcoming, the counterbalancing effect of the opposing conical portions renders such bearings incapable of gap adjustment once assembled. Stated another way, the inner component is incapable of axial displacement relative to the outer component due to the fact that the opposing conical pairs will ultimately cause the inner component to reach a position of equilibrium relative to the outer component that yields a zero net axial force.
As a second shortcoming, conical bearings of the type as described above have been found to be considerably difficult and expensive to manufacture and assemble. In particular, the dual-conical shape of the inner component typically necessitates that the outer component be constructed as multiple, separate pieces that are subsequently fused together once the inner component is properly positioned therewithin. Furthermore, this multi-stepped assembly process requires that the inner and outer components be precisely aligned.
As a third shortcoming, conical bearings of the type as described above have been found to perform poorly due to the occurrence of resonance. Specifically, due to the low viscosity of certain fluids, such as pressurized gas, resonance between components can be easily introduced into the air bearing as the fluid enters into the air bearing gap.